3. • INTRODUCTION
Plastids are large cytoplasmic organelles.
Plastids are major organelles found in the cells of
plants and algae.
Plastids are the site of manufacture and storage of
important chemical compounds used by the cell.
Plastids often contain pigments used in
photosynthesis and the types of pigments present
can change or determine the cell’s colour.
4. The term plastid was derived from the greek
word plastikas meaning formed or moulded.
This term was coined by schimper in 1885.
In plants, plastids may differentiate into
several forms, depending upon which function
they need to play in the cell.
The plastids are broadly classified into two
main types namely chromoplasts and
5. Some of the most common plastids include :
• TYPES AND FUNCTIONS OF PLASTIDS
Chromoplasts are what the name
describes, a place for the pigments to
be stored and synthesized in the
These are found in flowering plants,
fruits, and aging leaves.
8. The chloroplasts actually convert over to
There are carotenoid pigments here that
allow for the different colours you see in
fruits and the fall leaves.
One of the main reasons for these
structures and the colours is to attract
Gerontoplasts are basically chloroplasts that
are going through the aging process.
These are chloroplasts of the leaves that are
beginning to convert into different organelles
or are being repurposed, since the leaf is no
longer utilizing photosynthesis ( such as in
the fall months ).
Leucoplasts are the non-pigmented organelles.
Unlike the others we have talked about,
leucoplasts have no colour at all.
They are found in the non-photosynthetic
parts of the plants, such as the roots.
Depending on what the plant needs, they may
become essentially just storage sheds for
starches, lipids, and peoteins.
11. They are more readily used for synthesizing
amino acids and fatty acids.
Leucoplasts are further subdivided into three
different plastids :
12. Amyloplasts are the largest of the three
and are charged with storing starch.
Then there are the proteinoplasts that
help to store the proteins that a plant
needs and are typically found in seeds.
Finally, the elaioplasts are used to store
fats and oils that are needed by the
plant, specifically in seeds.
13. • CHLOROPLASTS
The chloroplasts are probably the most known
of the plastids. These are responsible for
photosynthesis. The chloroplast is filled with
thylakoids, which is where photosynthesis occurs.
14. Chloroplasts are organelles found in plant cells and
other eukaryotic organisms that conduct photosynthesis.
The word chloroplast is derived from the greek words
chloros, which means green, and plast, which means form
Chloroplasts are members of a class of organelles known
chloroplast varies in shape. They are spheroid or
ovoid or discoid in higher plants. They are cup-shaped in
chlamydomonas and spirally coiled in spirogyra.
15. • Size :
• The size of the plastids varies from species to
species. But the size remains constant for a given
call type. In higher plants, it is 4-5 microns in length
and 1-3 microns in thickness. Generally chroloplasts
of plants growing in shady places are larger in size.
• Number :
• The number of chloroplasts varies from plant to
plant, but it remains constant for a given plant. In
higher plants there are 20 to 40 chloroplasts per cell
or upto 1000 chloroplasts.
16. • structure :
Plant chloroplasts are large organelles that, like
mitochondria, are bounded by a double membrane
called the chloroplast envelope.
In addition to the inner and outer membranes of the
envelope, chloroplasts have a third internal membrane
system, called the thylakoid membrane.
The thylakoid membrane forms a network of flattened
discs called thylakoids, which are frequently arranged
in stacks called grana.
Grana are interconnected by branching membraneous
tubules called frets ( stromal lamellae ).
17. Because of this three-membrane structure, the internal
organization of chloroplasts is more complex than that
In particular, their three membranes divide chloroplasts
into three distinct internal compartments:
1. The intermembrane space between the
membranes of the chloroplast envelope
2. The stroma, which lies inside the envelope but outside
the thylakoid membrane
3. The thylakoid lumen.
18. A thylakoid has a flattened disk shape. Inside it is
an empty area called the thylakoid space or lumen.
Photosynthesis takes place on the thylakoid
membrane; as in mitochondrial oxidative
phosphorylation, it involves the coupling of cross-
membrane fluxes with biosynthesis via the
dissipation of a proton electrochemical gradient.
Embedded in the thylakoid membrane are antenna
complexes, each of which consists of the light-
absorbing pigments, including chlorophyll and
carotenoids, as well as proteins that bind the
pigments. These complexes are called as
20. Chloroplast contains proteins, lipids, carbohydrates, DNA,
RNA, carotenoids, chlorophyll and minerals. Composition
of these chemical were indicated in the following table :
S.NO. Chemical Percentage
1. Proteins 35 - 55%
2. Lipids 20 - 30%
3. Carbohydrates Variable
4. Chlorophyll 9%
5. Carotenoids 4.5%
6. RNA 3 - 4%
7. DNA 0.5%
8. Minerals 0.2%
21. This complex both increases the surface area
for light capture, and allows capture of
photons with a wider range of wavelengths.
The energy of the incident photons is absorbed
by the pigments and funneled to the reaction
centre of this complex through resonance
Two chlorophyll molecules are then ionised,
producing an excited electron, which then
passes onto the photochemical reaction centre.
22. Function of chloroplast
1. Chloroplast are the centres of synthesis and
metabolism of carbohydrates. During photosynthesis,
carbon dioxide and water are converted into organic
substances ( sugars, polysaccharides, fats and amino
acids ) in the presence of light :
Thus can be represented by general equation :
23. When hexose sugar is formed the equation is
as follows :
Photosynthesis consists of a primary light
reaction and a dark reaction ( which is
independent of light ).
24. During the primary light reaction, ‘ photolysis’ or
splitting of water takes place.
The hydrogen causes the reduction of an
acceptor such as NADP.
During the dark reaction the reduced acceptor
NADPH is utilized for the reduction of CO2 to a
primary product which eventually becomes a
carbohydrate. During the light reaction ADP is
phosphorylated to ATP. ( photophosphorylation )
25. 2. The similarities which chloroplast has
with the mitochondria as regards structure
and function, led to believe that they
contain the enzymes for the kreb’s cycle
and for the synthesis of fatty acids.
Chloroplast actively incorporates amino
acids in the presence of ATP and capable of
a certain degree of protein synthesis.